This disclosure relates generally to an apparatus and method for ventilating lungs of a subject enabling to choose one of a machine ventilation for assisting breathing functions and a manual ventilation with a manual bag which can be compressed for an inspiration.
Presently, anesthesia machines are optimized for delivering anesthesia to a patient using volatile anesthetic agent liquids. In such systems, the anesthetic agent is vaporized and mixed into the breathing gas stream in a controlled manner to provide a gas mixture for anesthetizing the patient for a surgical operation. The most common volatile anesthetic agents are halogenated hydrocarbon chains, such as halothane, enflurane, isoflurane, sevoflurane, and desflurane. Additionally, nitrous oxide (N2O) can be counted in this group of volatile anesthetic agents, although the high vapor pressure of nitrous oxide causes nitrous oxide to vaporize spontaneously in the high pressure gas cylinder, where it can be directly mixed with oxygen. The anesthetizing potency of nitrous oxide is seldom enough to anesthetize a patient and therefore is typically mixed with another volatile agent.
Since volatile anesthetic agents are expensive and environmentally damaging to the atmospheric ozone layer, anesthesia machines have been developed to minimize the use of these gases. To keep patient's anesthetized, a defined brain partial pressure for the anesthetic agent is required. This partial pressure is maintained by keeping the anesthetic agent partial pressure in the lungs adequate. During a steady state, the lung and body partial pressures are equal, and no net exchange of the anesthetic agent occurs between the blood and the lungs. However, to provide oxygen and eliminate carbon dioxide, continuous lung ventilation is required.
Anesthesia machines designed to deliver volatile anesthetic agents are designed to provide oxygen to the patient and eliminate carbon dioxide, while preserving the anesthetic gases. These goals are typically met using a re-breathing circuit, where an exhaled gas is reintroduced into the inhalation limb leading to the patient. In such a re-breathing circuit, carbon dioxide is absorbed from the expired gases by a carbon dioxide absorber. Before inhalation by the patient, the inhalation gas is supplied with additional oxygen and vaporized in aesthetic agents based upon the patient demand. In this arrangement, the additional gas flow added to the re-breathing circuit can be less than 0.5 L/min although the patient ventilation may be 5-10 L/min. The positive pressure inspiration is typically carried out using a ventilator, which is typically gas driven. In these ventilators, the patient breathing gas is pressurized by controlling a ventilator drive gas flow through a separate system maintaining the breathing gas separate from the ventilator drive gas. Such gas separation system may have form of a long reciprocating gas column or a physical flexible barrier construction.
Intravenously administered drugs provide an alternative to the volatile anesthetic agents. When an intravenous anesthesia is utilized, the primary functionality of the anesthesia re-breathing circuit is no longer needed, since the vaporized anesthetic agent is no longer circulating with the breathing gases. When intravenously administered anesthetic drugs are utilized, the anesthesia machine may use an open breathing circuit where a mixture of fresh oxygen and nitrogen is provided at the rate required by the patient and the expired gases can be removed from the circulation. In such an open system, carbon dioxide absorption is no longer needed since the re-circulation has been eliminated. Further, the isolation between the patient gases and the drive gases are no longer needed when the ventilation gases are provided directly to the patient. Thus, an anesthesia ventilator optimized for the intravenous anesthesia does not need the gas separation system and the carbon dioxide absorber. Further, a vaporizer for the volatile anesthetic agents is also no longer needed. These simplifications provide advantages in equipment size, eliminate much of the cleaning requirements by reducing the number of contaminated components, and streamlines the anesthesia machine manufacturing process.
Independently of the anesthesia practice, anesthesia ventilation involves ability for the clinician to manually ventilate the patient. This functionality is typically utilized during an anesthesia induction, weaning the patient from the anesthesia and ventilator, in assistance of spontaneous breathing and for the lung recruitment.
A desired property of manual ventilation system is given haptic feedback of the patient breath volume. Such feedback is achieved when the patient exhalation volume is collected to the manual bag, which is done in the following state of the art solutions.
The currently most used arrangement in ventilating manually is to have a breathing system equipped with an APL (Airway Pressure Limiting) valve. When using an airway pressure limiting valve the operating principle is that the valve is set to an pre-determined setting and when the manual ventilation bag is squeezed the gas volume is initially delivered to the patient, but when the APL pressure limit is reached the valve starts to bleed gas out from the breathing system. The valve will form a resistance and as the bag is further squeezed some of the volume will go to the patient and some will bleed through the valve. The volume going to the patient, if any, is not possible to determine haptically by how much the bag is squeezed. Neither the patient flow resistance can be identified since that is parallel to the APL valve bleeding resistance. If the patient is not ventilated by squeezing the bag, fresh gas or ventilator bias flow will increase the pressure to the APL limit leading to a sustained pressure and possible barotrauma or volutrauma. It is not possible to deliver a PEEP (Positive End Expiratory Pressure) to the patient with the APL valve.
Further development of mechanical manual ventilation valves has been done. An example of such valve is the “Berner valve”. This valve controls the breathing circuit and patient pressure at on low level during expiration and closes the valve during inspiration. The switchover between the phases is triggered with gas impulses caused by squeezing and releasing of the manual bag. To ventilate using this valve, both the bag compression and release actions need to be rigorous enough to generate the required impulse. Yielding from this, the Berner valve involves a safety problem related to every manual breath: Would the sensing of expiration fail, the valve remains closed resulting to unlimited breathing circuit and patient pressure increase. This problem related to the difficult use of the method, which has limited its clinical use.
The explained solutions represent a separate pressure control during the manual ventilation and mechanical ventilation most often controlled electronically using another pressure control valve. Also a solution using the same pressure control valve in both ventilation modalities is known. Because of the electrical breathing circuit and patient pressure control, the control algorithm can follow either the “APL” or “Berner” principle. Ability to set a maximum pressure limit solves the disadvantage of the “Berner” method. Also the minimum pressure during expiration (PEEP) can be controlled according to clinical demand.
For identification of the breathing phase, a predetermined control rule can be used. The controller of this system compares the measured breathing circuit pressure and/or breathing circuit flow pattern with the predetermined control rule, and based on this comparison, determines whether the manual breath cycle is inspiration or expiration: Compression of the manual bag increases the breathing circuit pressure and causes gas flow in the breathing circuit towards the patient. Respectively, release of the manual bag causes breathing circuit pressure decrease and breathing circuit flow from the patient towards the manual bag
Problem of the described system is that at the time the manual bag compression is started, the breathing circuit pressure control valve is open, and the compression necessarily does not yield to the breathing circuit pressure and/or flow pattern expected from the predetermined control rule. Therefore, even here the initial compression must be strong enough to cause the expected changes despite of the adjacent open pressure control valve.